In operation of a telephone system there are many instances where it is advantageous to power equipment which is remote from the central office by a battery which is also located remotely from the central office. For uninterrupted service it is desirable to automatically charge the remote battery. This is typically accomplished by drawing current from the subscriber's transmission line.
Battery charging control circuits which have previously been proposed customarily connect the remote battery directly to the telephone transmission line which, in turn, connects the subscriber's telephone to a central office. With these prior circuits current is drawn from the central office. With these prior circuits current is drawn from the central office d.c. power source to charge the remote battery.
Some attempts at charging the remote battery in this manner have been unsatisfactory because they resulted in distortion of dialing pulses or caused lock-up of certain telephone system circuits. Dialing pulse distortion occurs when significant battery charging current is drawn from the telephone transmission circuit during the dialing sequence or interval. Lock-up of the telephone circuit may occur when there is an excessive current drain from the telephone circuit at the time when the subscriber hangs up or very shortly after the subscriber hangs up.
The practice of drawing battery-charging current from the telephone transmission circuit has also created a significant problem in making line insulation tests with such currently used automatic testing equipment as the Western Electric ALIT (Automatic Line Insulation Testing). In an automatic line insulation test the transmission line to be tested is switched from the central office equipment to the test equipment to check the telephone transmission for current leakage. When test equipment such as the Western Electric ALIT is used, however, prior battery charging circuits which are known to applicant are not effective to prevent battery-charging current from being drawn from the transmission line for the duration of the test. As a result, the ALIT test equipment would yield erroneous results for tests on lines that are connected to the prior battery charging circuits.
In an effort to avoid dialing pulse distortion and circuit lock-up certain proposals for battery charging circuits have been made in U.S. Pat. No. 3,510,584 which issued to L. Q. Krasin et al on May 5, 1970. However, when such test equipment as the Western Electric ALIT is used, none of the three battery charging circuits described in this patent adequately prevents battery charging current from being drawn from the telephone transmission line during a line insulation test.
Furthermore, the first two battery charging circuit embodiments in U.S. Pat. No. 3,510,584 will not be adequately effective to prevent circuit lock-up if the voltage, which is applied to the battery charging circuit when the subscriber's telephone is off-hook, is relatively high. The magnitude of this applied voltage increases as the length of the transmission line between the battery charging circuit and the central office is decreased.
The third battery charging circuit embodiment in U.S. Pat. No. 3,510,584 attempts to avoid interference with the normal operation o the telephone transmission circuit by minimizing the current drain on the telephone transmission circuit. This embodiment, however, suffers from the disadvantage in that it causes a continuous current drain which is not enough to effect reliable operation for all expected conditions.
Furthermore, none of the three battery charging circuits in U.S. Pat. No. 3,510,584 is constructed to accommodate a battery charger. Use of a battery charger affords the advantage of charging the battery with a current magnitude that is greater than current magnitude which is drawn from the power supply source.